1. Field of the Invention
This invention relates to a charge multiplying solid-state electronic image sensing device, a method of controlling the device, and an endoscope apparatus that uses this charge multiplying solid-state electronic image sensing device.
2. Description of the Related Art
A charge multiplying solid-state electronic image sensing device has a charge multiplier possessing a number of stages. By applying charge multiplying drive pulses to the charge multiplier, electric charge that has accumulated in photodiodes is multiplied and output.
The overall multiplication factor of the charge multiplier is decided by the charge multiplication factor per stage and the total number of stages of the charge multiplier. This means that a fluctuation in the charge multiplication factor per stage appears as a large fluctuation in the overall multiplication factor of the charge multiplier. For example, assume that the charge multiplication factor per stage is 1.00854 and that the number of stages of the charge multiplier is 400. In order to hold a variation in the overall multiplication factor of the charge multiplier to a value below several percent, it is required that the fluctuation in the charge multiplication factor per stage be held to a small 0.1% owing to a fluctuation in the charge multiplying drive pulses applied to the charge multiplier. Suppressing a fluctuation in the charge multiplication factor per stage to 0.1% is very difficult.
A charge multiplying solid-state electronic image sensing device sometimes is utilized in an endoscope apparatus owing to its ability to detect extremely weak light. In an endoscope apparatus, internal tissue is illuminated by white light in an ordinary observation mode until the tip of the scope reaches internal tissue deemed to be the affected area. When the internal tissue deemed to be the affected area is ascertained, the mode is switched over to a fluorescent observation mode and the internal tissue is illuminated with excitation light. As compared with the ordinary observation mode, in the fluorescent observation mode an extremely faint fluorescent image must be sensed. For this reason, a charge multiplying solid-state electronic image sensing device that is capable of detecting very faint light is used in an endoscope.
In an endoscope apparatus, the level of the charge multiplying drive pulses is changed in response to mode changeover. That is, in the fluorescent observation mode, the level of the charge multiplying drive pulses is raised to increase the multiplication factor because only a small amount of signal charge can be stored and, hence, the video signal level is low in this mode. When the level of the charge multiplying drive pulses is changed, however, a fluctuation is produced in the charge multiplying drive pulses at the time of the change. As a consequence, a variation in the overall multiplication factor of the charge multiplier cannot be ignored.